Add-ons

Firefox is well known for its large library of add-ons which can be used to add new features or modify the behavior of existing features. Firefox's "Add-ons Manager" is used to manage installed add-ons or find new ones.

Plugins

in the Firefox address bar or go to the Add-ons entry in the Firefox Menu and select the Plugins tab.

Configuration

Firefox exposes a number of configuration options. To examine them, enter in the Firefox address bar:

about:config

Once set, these affect the user's current profile, and may be synchronized across all devices via Firefox Sync. Please note that only a subset of the about:config entries are synchronized by this method, and the exact subset may be found by searching for services.sync.prefs in about:config. Additional preferences and third party preferences may be synchronized by creating new boolean entries prepending the config value with services.sync.prefs.sync (documentation is still applicable.) To synchronize the whitelist for the extension NoScript:

services.sync.prefs.sync.capability.policy.maonoscript.sites

The boolean noscript.sync.enabled must be set to true to synchronize the remainder of NoScript's preferences via Firefox Sync.

Firefox also allows configuration for a profile via a user.js file: user.js kept in the profile folder, usually ~/.mozilla/firefox/xxxxxxxx.default/. For a useful starting point, see e.g custom user.js which is targeted at privacy/security conscious users.

One drawback of the above approach is that it is not applied system-wide. Furthermore, this is not useful as a "pre-configuration", since the profile directory is created after first launch of the browser. You can, however, let firefox create a new profile and, after closing it again, copy the contents of an already created profile folder into it.

Sometimes it may be desired to lock certain settings, a feature useful in widespread deployments of customized Firefox. In order to create a system-wide configuration, follow the steps outlined in Locking preferences:

Firefox uses PulseAudio for audio playback and capture. For sound to work, you need to install the pulseaudio package.

In case, for whatever reason, PulseAudio is not an option for you, you can use apulse instead. To make this work, it is necessary to exclude /dev/snd/ from Firefox' sandboxing by adding it to the comma-separated list in about:config:

Open With extension

Proceed with instructions to install a file in your system and test the installation.

Click Add browser.

In the dialog write a name for this menu entry and command to start a video streaming capable player (e.g. /usr/bin/mpv).

(Optional step) Add needed arguments to the player (e.g. you may want --force-window --ytdl for mpv)

Right click on links or visit pages containing videos. Select newly created entry from Open With's menu and if the site is supported, the player will open as expected.

The same procedure can be used to associate video downloaders such as youtube-dl.

Spell checking

Install the hunspell package. You also need to install dictionaries for your language, such as hunspell-fr (for the French language) or hunspell-he (for Hebrew).

To enable spell checking for a specific language right click on any text field and check the Check Spelling box. To select a language for spell checking to you have right click again and select your language from the Languages sub-menu.

To get more languages just click Add Dictionaries... and select the dictionary you want to install from the list. For Russian there is firefox-spell-ru.

To make the left mouse button warp the scrollbar instead of the middle one on KDE, go to System Settings > Application Style > GNOME/GTK Application Style and choose 'Jump to the mouse cursor position' in the 'On left-clicking the scroll bar' option.

To use the KDE file selection and print dialogs in Firefox 64 or newer:

mozilla-extension-gnome-keyring-gitAUR (all-JavaScript implementation) to integrate Firefox with GNOME Keyring. To make firefox-gnome-keyring use your login keychain, set extensions.gnome-keyring.keyringName to login in about:config. Note the lowercase 'l' despite the the keychain name having an uppercase 'L' in Seahorse.

Smooth Scrolling

In order to get smooth physics-based scrolling in Firefox, the general.smoothScroll.msdPhysics configurations can be changed to emulate a snappier behaviour like in other web browsers. For a quicker configuration, append the following to ~/.mozilla/firefox/your-profile/user.js (requires restart):

Screenshot of webpage

You can Take a Screenshot by either clicking the Page actions button (the three horizontal dots) in the address bar or by right-clicking on the webpage. See [1] for more information.

Note:

The Save button misleadingly uploads your screenshot to a firefox.com subdomain. Set extensions.screenshots.upload-disabled to true to disable this behaviour. [2]

If privacy.resistFingerprinting is enabled, to take a screenshot of a website using the above method, you need to grant it Extract Canvas Data permission.

As an alternative you can use the full-page screenshot button in the Developer Tools, which you can open with F12 or Ctrl+Shift+i (you might need to enable the button first in the Developer Tools Settings > Available Toolbox Buttons > Take a screenshot of the entire page).

Window manager rules

To apply different configurations to Firefox windows, change the WM_CLASS string by using Firefox's --class option, to a custom one.

Profiles

To start new Firefox instances, multiple profiles are required. To create a new profile:

$ firefox [--new-instance] -P

Class can be specified when launching Firefox with a not-in-use profile:

$ firefox [--new-instance] -P profile_name --class=class_name

Touchscreen gestures and pixel-perfect trackpad scrolling

To enable touch gestures (like scrolling and pinch-to-zoom) and one-to-one trackpad scrolling (as can be witnessed with GTK3 applications like Nautilus), set the MOZ_USE_XINPUT2=1environment variable before starting Firefox.

New tabs position

To control where new tabs appears (relative or absolute), use browser.tabs.insertAfterCurrent and browser.tabs.insertRelatedAfterCurrent. See [3] for more informations.

Troubleshooting

All Firefox extensions disabled (May 2019)

Warning:Do not delete and re-install your addons to try and fix the issue, as you will lose your addons' data.

Between 3–5 May 2019, all Firefox addons (web extensions, themes, search engines, and language packs) were mistakenly marked as Legacy addons, and could not be enabled. The bug was caused by an expired intermediate certificate.

For most users, the issue has been fixed in Firefox 66.0.4 and Firefox ESR 60.6.2. The addons should be automatically re-enabled as soon as the patched version of Firefox is installed. Certain users, such as those who have set a Master Password, might require extra steps. Please see Firefox's official support page for more information.

Font troubleshooting

Firefox has a setting which determines how many replacements it will allow from fontconfig. To allow it to use all your replacement-rules, change gfx.font_rendering.fontconfig.max_generic_substitutions to 127 (the highest possible value).

Setting an email client

Inside the browser, mailto links by default are opened by a web application such as Gmail or Yahoo Mail. To set an external email program, go to Preferences > Applications and modify the action corresponding to the mailto content type; the file path will need to be designated (e.g. /usr/bin/kmail for Kmail).

Outside the browser, mailto links are handled by the x-scheme-handler/mailto mime type, which can be easily configured with xdg-mime. See Default applications for details and alternatives.

File association

Firefox keeps creating ~/Desktop even when this is not desired

Firefox uses ~/Desktop as the default place for download and upload files. To change it to another folder, set the XDG_DESKTOP_DIR option as explained in XDG user directories.

Make plugins respect blocked pop-ups

Some plugins can misbehave and bypass the default settings, such as the Flash plugin. You can prevent this by doing the following:

Type about:config into the address bar.

Right-click on the page and select New > Integer.

Name it privacy.popups.disable_from_plugins.

Set the value to 2.

The possible values are:

0: Allow all popups from plugins.

1: Allow popups, but limit them to dom.popup_maximum.

2: Block popups from plugins.

3: Block popups from plugins, even on whitelisted sites.

Middle-click behavior

To use the middle mouse button to paste whatever text has been highlighted/added to the clipboard, as is common in UNIX-like operating systems, set either middlemouse.contentLoadURL or middlemouse.paste to true in about:config. Having middlemouse.contentLoadURL enabled was the default behaviour prior to Firefox 57.

To scroll on middle-click (default for Windows browsers) set general.autoScroll to true.

Backspace does not work as the 'Back' button

According to MozillaZine, the Backspace key was mapped based on which platform the browser was running on. As a compromise, this preference was created to allow the Backspace key to either go back/forward, scroll up/down a page, or do nothing.

To make Backspace go back one page in the tab's history and Shift+Backspace go forward, set browser.backspace_action to 0 in about:config.

To have the Backspace key scroll up one page and Shift+Backspace scroll down one page, set browser.backspace_action to 1. Setting this property to any other value will leave the key unmapped (Arch Linux defaults to 2, in other words, it is unmapped by default).

Firefox does not remember login information

It may be due to a corrupted cookies.sqlite file in Firefox's profile folder. In order to fix this, just rename or remove cookie.sqlite while Firefox is not running.

Open a terminal of choice and type the following:

$ rm -f ~/.mozilla/firefox/<profile id>.default/cookies.sqlite

The profile id is a random 8 character string.

Restart Firefox and see if it solved the problem.

"Do you want Firefox to save your tabs for the next time it starts?" dialog does not appear

Firefox detects the wrong version of my plugin

When you close Firefox, the latter saves the current timestamp and version of your plugins inside pluginreg.dat located in your profile folder, typically in ~/.mozilla/firefox/xxxxxxxx.default/.

If you upgraded your plugin when Firefox was still running, you will thus have the wrong information inside that file. The next time you will restart Firefox you will get that message Firefox has prevented the outdated plugin "XXXX" from running on ... when you will be trying to open content dedicated to that plugin on the web. This problem often appears with the official Adobe Flash Player plugin which has been upgraded while Firefox was still running.

The solution is to remove the file pluginreg.dat from your profile and that is it. Firefox will not complain about the missing file as it will be recreated the next time Firefox will be closed. [4]

JavaScript context menu does not appear on some sites

You can try setting dom.w3c_touch_events.enabled to 0 in about:config.

Firefox does not remember default spell check language

The default spell checking language can be set as follows:

Type about:config in the address bar.

Set spellchecker.dictionary to your language of choice, for instance en_GB.

Notice that the for dictionaries installed as a Firefox plugin the notation is en-GB, and for hunspell dictionaries the notation is en_GB.

When you only have system wide dictionaries installed with hunspell, Firefox might not remember your default dictionary language settings. This can be fixed by having at least one dictionary installed as a Firefox plugin. Notice that now you will also have a tab Dictionaries in add-ons.

Tearing video in fullscreen mode

Firefox WebRTC module cannot detect a microphone

WebRTC applications for instance Firefox WebRTC getUserMedia test page say that microphone cannot be found. Issue is reproducible for both ALSA or PulseAudio setup. Firefox debug logs show the following error: